This invention relates to a reflection sound generator for generating reflection sounds of an input sound signal using an FIR (Finite Impulse Response) filter. The reflection sound generator allows a greater number of reflection sounds to be generated from a smaller number of parameters, and the reflection sound generator further makes it easy to set and change reverberation characteristics of a sound field such as room size and liveness.
The reflection sound generator using the FIR filter is a device to carry out convolution operation of an input sound signal based on reflection sound parameters, which determine a sequence of or group of reflection sounds in terms of respective delay times and gains or magnitudes, to produce reflected and reverberated sounds. Such a device is used in various applications, for example, for creating the realism of any sound field space (e.g., a hall space) in a limited space (e.g., small room). It may also be provided in an acoustic feedback system for adjusting reverberation characteristics of a room (e.g., a music listening room) or a hall, or for prolonging a reverberation time.
FIG. 2 shows the most basic structure of a reflection sound generator using a conventional FIR filter. This device is designed to produce reflection sounds through one FIR filter 10. For the FIR filter 10, as shown in FIG. 3(a), for example, a set of reflection sound parameters is installed such that a sequence of reflection sounds are generated at proper time intervals while their magnitudes gradually decrease to attenuate with time. The FIR filter 10 carries out the convolution operation of an input sound signal based of the reflection sound parameter set to generate a corresponding reflection sound signal.
The time interval At tuned by the FIR filter 10 can be lengthened or shortened throughout its full time domain, thereby causing a variation of room size. For example, as shown in FIG. 3(b), the time interval At of the FIR filter 10 can be lengthened so that the audience can feel the room to become wider. Gains of the reflection sound parameters can also be changed in proportion to the delay times of the respective reflection sounds while maintaining the time interval At, resulting in a variation of liveness. For example, as shown in FIG. 3(c), each gain can be raised in proportion to each delay time to create a live sound field.
According to the structure of the reflection sound generator of FIG. 2, the number of parameters set for the FIR filter 10 is, however, required to correspond to the number of reflection sounds (equivalent to the number of taps of the device) to be generated. To solve this problem, another type of reflection sound generator such as shown in FIG. 4 has been proposed. In this reflection sound generator, a plurality of FIR filters 12, 14, 16 are connected in series while respective outputs of the FIR filters 12, 14, 16 are coupled commonly to an adder 17 to generate resultant output sounds. FIGS. 5(a), 5(b) and 5(c) show reflection sound-parameters of the FIR filters 12, 14, 16, respectively. As shown, the first FIR filter 12 generates a top part containing an initial reflection sound. Based on the initial reflection sound part, the second FIR filter 14 generates a reverberant reflection sound part following the initial reflection sound part. Then, based on the reverberant reflection sound part, the third FIR filter 16 generates a further reverberant reflection sound part following the reverberant reflection sound part. In this case, the next-stage reflection sounds are generated based on the previous-stage reflection sounds, so that the density of reflection sounds gradually increases along the time axis, thereby generating a greater number of reflection sounds from a smaller number of parameters.
The structure of the reflection sound generator of FIG. 4, having the reflection sound parameters shown in FIGS. 5(a), 5(b) and 5(c), however, raises another problem with respect to setting and changing of reverberation characteristics such as the room size and the liveness. In other words, the setting and changing of reverberation characteristics requires respective parameters to be adjusted for each FIR filter 12, 14, 16, and this adjustment.makes it hard to freely set and change the reverberation characteristics.
The present invention has been made to solve the problems in the conventional technology, and it is an object of the invention to provide a reflection sound generator that allows a greater number of reflection sounds to be generated from a smaller number of parameters, and that further makes it easy to set and change reverberation characteristics such as room size and liveness.
The inventive apparatus is constructed for processing an input sound to generate a sequence of reflection sounds along a time axis including an initial reflection sound and subsequent reverberant reflection sounds. In the inventive apparatus, a first filter of Finite Impulse Response type is provided with a first set of parameters representing a first distribution pattern of reflection sounds. The first distribution pattern has a time length sufficient to cover the initial reflection sound and the subsequent reverberant reflection sounds. Each parameter determines a magnitude (gain) and a delay time of each reflection sound such that the reflection sounds are distributed at intervals along the time axis and such that the magnitudes of the reflection sounds gradually decrease along the time axis. The first filter executes convolution operation of sample data of the input sound by the first set of parameters to generate first data containing a sequence of reflection sounds of the input sound. A second filter of Finite Impulse Response type is provided with a second set of parameters representing a second distribution pattern of additional reflection sounds. The second distribution pattern has a time length shorter than that of the first distribution pattern and longer than each interval of the reflection sounds. Each parameter determines a magnitude and a delay time of each additional reflection sound such that the additional reflection sounds are arranged at intervals shorter than those of the reflection sounds. The second filter executes convolution operation of the first data by the second set of parameters to generate second data containing additional reflection sounds which fill the intervals of the reflection sounds of the input sound.
Preferably, the second filter is provided with the second set of parameters designed such that the magnitudes of the additional reflection sounds gradually decrease along the time axis.
Preferably, the first filter is provided with the first set of parameters designed such that the reflection sounds are distributed at variable intervals, which become gradually short along the time axis.
Preferably, the first filter is provided with the first set of parameters, which can be altered to expand or contract the intervals of the reflection sounds while maintaining relative proportions of the magnitudes thereof so as to change.a rate of attenuation of the reflection sounds along the time axis.
Preferably, the first filter is provided with the first set of parameters, which can be altered to increase or decrease the magnitudes of the reflection sounds in proportion to the delay times of the reflection sounds while maintaining the intervals of the reflection sounds so as to change a rate of attenuation of the reflection sounds along the time axis.
Preferably, the first filter is provided with the first set of parameters, which can be altered so as to periodically fluctuate the delay times of the respective reflection sounds independently from each other along the time axis.
Preferably, the second filter is provided with the second set of parameters, which can be altered so as to periodically fluctuate the delay times of the respective additional reflection sounds independently from each other along the time axis.
Preferably, the first filter is provided with the first set of parameters containing a first direct parameter effective to generate a first direct sound identical to the input sound in precedence to the sequence of the reflection sounds of the input sound by a first lead interval, and the second filter is provided with the second set of parameters containing a second direct parameter effective to generate a second direct sound identical to the first direct sound in precedence to the additional reflection sounds by a second lead interval which is set comparable to the first lead interval.
According to the present invention, there is provided the reflection sound generator using the series connection of the first FIR filter and the second FIR filter. For the first FIR filter (main filter), the first set of reflection sound parameters is set in correspondence to a reflection sound group being generated throughout the entire time domain covering the initial reflection sound and the subsequent reverberant reflection sounds at relatively scattered and irregular time intervals, and being attenuated as delay time elapses. Namely, the first FIR filter carries out the convolution operation of the sample data of the input sound signal based on the first reflection sound parameter set to generate the first reflection sound data. For the second FIR filter (supplementary filter), the second set of reflection sound parameters is set in correspondence to another reflection sound group being generated at irregular but denser time intervals than those of the reflection sound group generated by the first reflection sound parameter set. The overall time length of the second reflection sound group is set longer than any of reflection sound generation intervals of the first reflection sound group and shorter than the overall time length of the first reflection sound group. The second FIR filter carries out the convolution operation of the first reflection sound data generated through the first FIR filter based on the second reflection sound parameter set so as to generate the second reflection sound data for filling out the reflection sound generation intervals of the first FIR filter.
According to the present invention, the second FIR filter produces the additional reflection sounds based on the reflection sounds produced through the first FIR filter, to fill out the reflection sound generation intervals of the first FIR filter. This makes it possible to generate a greater number of reflection sounds from a smaller number of parameters. Further, the first reflection sound parameters produce the sequence of reflection sounds throughout the entire time domain covering the initial reflection sound and the later reverberant reflection sounds, so that the total attenuation characteristics of the reverberation can be mainly determined based on the setting of the first FIR filter. Thus, the reverberation characteristics such as room size and liveness can be set and changed by the first FIR filter alone, and this makes it easy to set and change the reverberation characteristics.
The second reflection sound parameters may be set such that the reflection sound group is attenuated as the delay time elapses. Further, the first reflection sound parameters may be set such that reflection sound generation intervals are gradually shortened as the delay time elapses. Such setting makes it possible to obtain more natural reverberation. Furthermore, the first reflection sound parameters alone or both the first and second reflection sound parameters may be set such that the delay time of each reflection sound is fluctuated with time in each individual cycle, thereby creating random fluctuations to the reflection sounds to prevent occurrence of coloration.
Direct sound may be output without passing the first and second FIR filters. Otherwise, The direct sound may be output through the first and second FIR filters. In such a case, the first direct sound parameter is arranged before the first reflection sound parameter set of the first FIR filter so that the input sound signal is output as it is with no time lag. On the other hand, the second direct parameter is arranged before the second reflection sound parameter set of the second FIR filter so that the direct sound data of the first FIR filter is further output from the second FIR filter as it is with no time lag. The second reflection sound parameter set is arranged after the second direct parameter with a certain lead time interval nearly equal to the lead time interval between the first direct sound parameter and the top of the first reflection sound parameter set.